Most traditional ceramic manufactured products, such as wall tiles and floor tiles, are made of a ceramic body that confers form and mechanical properties to the object; the ceramic body generally has some porosity and poor aesthetic qualities.
Said ceramic body, which is defined “green” or, alternatively, “fired”, if previously fired, is then usually coated with a ceramic layer, called ceramic glaze; the ceramic glaze is completely sintered by firing, in such a way to gain suitable superficial aesthetic qualities and, in the meantime, to become a fluid-proof barrier; as a matter of fact, after firing, the ceramic glaze has usually no porosity and it is generally resistant to abrasion and to the attack of chemical agents such as acids, bases, dyes.
The aesthetic finishing of the ceramic material can be completed by a decoration phase, that is by the application of sinterable and variously coloured ceramic materials which are applied according to a preset drawing (décor).
The décor can be applied either on green or fired ceramic body, on which the glaze was previously set, or, in the so called third firing decorations, after the firing, on the sintered glaze.
Different techniques are used to transfer images to the ceramic substrate: i.e. screen printing and photogravure (commonly referred to as rotocolor). These technologies require flat substrate or with minimum roughness and they are suitable for mass production, but have very limited flexibility of new design set up and changeover among designs.
Another technique of printing decoration on ceramics is digital printing by inkjet technique.
Digital printing and decoration by inkjet technique is widely used in multiple sectors, such as graphic arts, textile industry, industrial marking and it is well known, both referring to the printing equipments and also to the inks used.
Peculiarly in ceramic applications, the thermal treatment, which is required once the substrate has been printed, makes the conventional inks, that are used in the other applications and are mainly based on organic pigments, unsuitable for use.
Two kind of inks for inkjet printing of ceramics are known: inks constituted by solutions of metallic cations and inks based on dispersions of inorganic pigments.
As far as inks based on dispersions of inorganic pigments are concerned, it is mandatory that the inorganic pigments are well dispersed into the liquid medium and possess nano-scale dimensions, for the ceramic inkjet ink flows at high speed through the small nozzles of the print head (30-100 μm in diameter).
Nano-scale dispersions of the inorganic pigments are usually obtained by milling with microspheres the pre-dispersed pigments in the medium, in the presence of milling aids.
Examples of ceramic inkjet inks based on dispersions of inorganic pigments in polar organic mediums are described in EP 2159269, WO 2006/126189, EP 1840178; the inks are generically said to contain antisettling and/or dispersing agents.
Nonetheless, there is still the industrial need for improved ceramic inkjet inks based on inorganic ceramic pigments having low viscosity, particle size below 0.8 μm, long shelf life and suitable to be printed on ceramic surfaces and passed through a high temperature kiln to form a permanently sintered glazed print.
It has now been found that the reaction product between a polyethyleneimine and a 12-hydroxystearic acid and ε-caprolactone co-polyester can conveniently be used in the preparation of inkjet inks for ceramic inkjet printing machines. Surprisingly, the reaction product between a polyethyleneimine and a 12-hydroxystearic acid and ε-caprolactone co-polyester, is perfectly suitable, in the milling phase, to fluidize the pre-dispersed inorganic ceramic pigments, allowing their rapid milling and subsequently preventing agglomeration and sedimentation of the nano-scale inorganic ceramic pigments in the final inks. The reaction products between a polyethyleneimine and a 12-hydroxystearic acid polyester are known products that belongs to a wide class of dispersing agents obtained by amidation and/or salification of polyimines and carboxyl terminated polyesters. They have been described in many patents; by way of example we cite: U.S. Pat. No. 4,224,212, U.S. Pat. No. 4,861,380, U.S. Pat. No. 5,700,395, U.S. Pat. No. 6,197,877 and JP 63-197529.
JP 63-197529, in particular, describes dispersing agents obtained by reaction between a polyethylenimine and a block polyester. The block polyester derives from a reaction of a monocarboxylic acid with ε-caprolactone followed by a reaction with 12-hydroxystearic acid. Though according to the general description of JP 63-197529, the dispersing agent can contain a high number of 12-hydroxystearic acid residues (up to 10 moles of 12-hydroxystearic acid residues per mole of ε-caprolactone), the examples show only co-polyesters prepared by a mixture of acids that contains between 10% and 26% (w/w) of 12-hydroxystearic acid.
Such wide class of dispersing agents reported above, is generally suitable for use as dispersing agents for various solids in organic liquids.
However, none of the above mentioned documents hints that the reaction product between a polyethylenimine and a 12-hydroxystearic acid and ε-caprolactone co-polyester, can be used as dispersing agent in applications where the solid is an inorganic vitrifiable ceramic pigment with nano scale dimensions, as required for the preparation and stabilization of ceramic inkjet inks.
The patent application WO 2012/076438 reports a dispersing agent suitable for grinding and stabilizing ceramic inkjet inks and it is obtained by reaction between a polyethylenimine and a polyester derived from ricinoleic acid. In this patent application it is pointed out that a dispersing agent obtained from polyethylenimine and 12-hydroxystearic acid homo-polyester is by far less effective in the ceramic pigments milling at the nanoscale range and dispersing activity, than the dispersing agent derived from polyethylenimine and ricinoleic acid polyester.
Now we found that the reaction product between a polyethylenimine and a 12-hydroxystearic acid and ε-caprolactone co-polyester, in the peculiar application, gives better performance than those of analogous dispersing agents derived from ricinoleic acid instead that from 12-hydroxystearic acid, even when the weight percentage in 12-hydroxystearic acid used in the co-polymer synthesis is high.